1. Field of the Invention
The present invention relates to image processing, and more particularly to an image processing apparatus, an image forming apparatus, and a recording medium, for performing image reduction correction on image data by deleting pixels.
2. Description of the Related Art
In recent years and continuing, there are increasingly more demands for image forming apparatuses with higher performance. In order to meet demands for image forming processes performed with higher speed, higher precision, and higher quality, various problems are to be considered. For example, while performing automatic double-sided printing, a paper sheet may become slightly reduced in size due to the fixing/heating operation. As a result, the position of an image formed on the first side of the transfer sheet may be displaced from that on the second side of the transfer sheet.
Furthermore, in electrophotographic color printing operations, a tandem method is preferably applied as it is advantageous in terms of high speed. However, the positions of the images formed by the image forming units of the respective colors may be displaced from each other.
In order to mitigate degradations in the image quality caused by such positional displacements, there has been proposed a process of deleting pixels from the original image data before forming the image, in accordance with the detected amount of changes or the anticipated amount of changes in the sheet size, so that the image data can be corrected for achieving the desired image size. However, when there are cycles in the image forming operation, such as cycles for deleting pixels, the formed images may have degraded image quality. Examples of degraded image quality are streak-like irregularities and moiré that appear at particular spatial frequencies of the pixel deleting operation. Thus, there is a need for a measure to prevent such degraded image quality.
FIG. 14 schematically illustrates an example of an image reduction correction process performed by deleting pixels. FIG. 14 shows images corresponding to image data including two-dimensional arrangements of pixels, in which the pixels are represented by squares “□”. Furthermore, the hatched pixels correspond to pixels to be deleted. In FIG. 14, image data 610 is the data of the image before undergoing the image reduction correction process, and image data 600 is the data of the image that has been reduced in the sub scanning direction by deleting pixels. The image data 600 that has undergone the process is obtained by deleting pixels from the original image data 610 in such a manner that pixels of one scanning line are deleted at every eleventh scanning line. As the pixels are deleted, the subsequent pixels are shifted in the direction in which the image is reduced (reduction direction). Accordingly, the image data 600 is reduced by an amount of the deleted pixels (i.e., the number of pixels corresponding to the “reduction amount” in FIG. 14). However, in cases where the pixels are deleted at regular intervals as shown in the image data 600 that has undergone the process, the following problem may arise. That is, even if the positional displacement between the images can be corrected by deleting pixels, defective images may be formed due to the cycles in the pixel deleting operation. For example, there may be considerably visible streak-like irregularities in the density of the image, which are referred to as “banding”.
Banding is heavily dependent on the visual properties of the human being. It is known that density irregularities are most noticeable at a spatial frequency falling in a range of approximately 5 through 10 cycles/deg (cycles per degree of visual angle). This corresponds to a spatial frequency falling in a range of 0.8 through 1.6 cycles/deg when the observation distance is 350 (mm). For example, by deleting pixels of one scanning line at every 48th scanning line, the image can be reduced by approximately 2%. However, when the resolution is 1200 dpi (dots per inch), density modulation of approximately one cycles/deg occurs. Accordingly, considerably visible banding is expected to appear.
In relation to image correction, Japanese Laid-Open Patent Application No. 2001-5245 (patent document 1) discloses an electrophotographic color image forming apparatus including plural photoconductors. For the purpose of correcting the difference between the widths in the main scanning direction of images of the respective colors, this image forming apparatus includes a displacement amount detecting unit for detecting the displacement amount of widths of the toner images corresponding to plural colors which are transferred onto a transfer material, and a displacement amount correcting unit for adding pixels to each of the image data items corresponding to the plural colors to make the toner images of the plural colors have the same widths, based on the displacement amount detected by the displacement amount detecting unit. Patent document 1 also describes that the pixels are added at random intervals to prevent patterns from appearing in any direction, thereby achieving color images of high printing quality.
Furthermore, Japanese Laid-Open Patent Application No. 2005-117615 (patent document 2) discloses an image forming apparatus including a screen processing unit for performing a screen process on the image data and a position correction unit for correcting color displacement (which occurs due to positional displacement) by adding or deleting pixels to/from the image data at predetermined arrangements. The screen processing unit applies a different screen in accordance with properties of each image region included in the image. The position correction unit adds or deletes pixels in arrangement cycles that do not match these screens within a range of less than or equal to 0.5 mm in the printed image.
A method of evaluating the extent of banding is disclosed in “Evaluation of Density Variations for a Thermal Recording System” written by Junichi Yoneda, FUJIFILM RESEARCH & DEVELOPMENT, No. 42, pp. 40-47, 1997 (nonpatent document 1). Also, in relation to image correction, Japanese Laid-Open Patent Application No. 2007-8132 (patent document 3) discloses a method of correcting scanning irregularities and scanning widths.    Patent document 1: Japanese Laid-Open Patent Application No. 2001-5245 (Japanese Patent No. 3539283)    Patent document 2: Japanese Laid-Open Patent Application No. 2005-117615    Patent document 3: Japanese Laid-Open Patent Application No. 2007-8132    Nonpatent document 1: “Evaluation of Density Variations for a Thermal Recording System” written by Junichi Yoneda, FUJIFILM RESEARCH & DEVELOPMENT, No. 42, pp. 40-47, 1997
As described above, several techniques have been proposed for mitigating banding in images as a result of image zooming processes performed by deleting pixels. However, in the conventional technology disclosed in patent documents 1 and 2, complex computing procedures and large memory space are required in order to determine the preferable positions from which pixels are to deleted, which leads to increased costs.